The present invention relates to a synchronous motor drive apparatus for driving a synchronous motor with a pulse-duration modulation system, and more particularly to a synchronous motor drive apparatus which is designed to prevent adverse effects due to the dead zone of a pulse-duration modulation circuit provided to avoid short-circuiting an inverter circuit.
Apparatus for driving AC synchronous motors under variable-speed control are generally arranged such that a sine-wave current command is modulated by pulse-duration or pulse-width modulation to drive an inverter circuit, and the size wave thus modulated is supplied to the synchronous motor. The apparatus therefore employs a pulse-duration modulation drive circuit and the inverter circuit is driven thereby. With this arrangement, only the inverter circuit at the final stage needs to be constructed so as to withstand a high voltage, the circuit structure is simple, and control is relatively easy to carry out.
For a three-phase AC drive mode, the inverter circuit has a total of six transistors with two transistors for each phase. The transistors in each phase are controlled so that they are alternately turned on and off during opposite phases. To prevent the transistors in each phase from being rendered conductive at the same time, that is, to prevent a DC power supply for feeding the inverter circuit from being short-circuited due to the switching of the transistors, there is included a dead time in a signal modulated by way of pulse-duration modulation for driving the transistors. The dead time is established by providing a dead zone in the pulse-duration modulation circuit. The dead time in the modulated signal, however, is disadvantageous in that it causes the modulated sine-wave current to be distorted, allows level produced by the excitation to be increased, and gives rise to torque variations, failing to generate a required torque. Particularly, thereby when the motor rotates at low speed, the sine-wave current command is of low frequency and influences due to the dead zone are increased to the point where they cannot be neglected.